1. Field of the Invention
The present invention relates generally to a lens for a lighting device mounted on a vehicle. More particularly, the present invention relates to improvement of a lens of the foregoing kind which assures that a phenomenon of dewing arising due to quick cooling of the inner wall surface of the lens at the time of vehicle body washing or rainfall can properly be controlled.
2. Background Art
To facilitate understanding of the present invention, a typical conventional lens of the foregoing type will be described below with reference to FIG. 3.
As shown in the drawing, a lens 92 is arranged on the front side of a lighting device 90 mounted on a vehicle, and a venting tube 91 is attached to the rear surface of the lighting device 90. To cope with a phenomenon of dewing to be visually recognized on the lighting device 90, measures have been hitherto taken such that an inner diameter of the venting tube 91 is enlarged for the purpose of improving ventilation properties of the lighting device 90, and/or a thickness of the lens 92 is increased over the whole surface of the same so as to allow the interior of the lighting device 90 to be hardly affected by the exterior cooling action presently progressing on the front surface of the lens 92.
However, these measures hitherto taken for the lighting device 90 in that way are merely intended to delay the time limit that a phenomenon of dewing arises. Thus, once the cooling action progress in excess of this limit, any other measure can not be taken for the purpose of preventing an occurrence of dewing under severe outside conditions with the result that drops of dew are grown on the surface of the lens 92 as if no measure is taken. For example, when a vehicle body is quickly cooled by spraying a large quantity of water, a phenomenon of dewing is visually recognized on the surface of the lens 92 although a certain measure is practically taken against the phenomenon of dewing by using a rust-proofing agent or the like. Consequently, each driver uselessly feels unreliability as to whether a phenomenon of dewing arises with his vehicle or not.